Implantable medical devices (IMDs) such as pacemakers and implantable cardioverters/defibrillators (ICDs) typically have a non-rechargeable battery with an expected lifetime of 3-15 years, with 5-10 years being most common. This has been made possible with advancements in battery and capacitor technology, as well as reducing power requirements of the components within the device. At the same time, many more features, therapies and capabilities are provided in modern IMDs that simply require additional power.
Therefore, with these considerations in mind, power management is an important aspect in the design and manufacture of IMDs.
During recent years distance telemetry capabilities have been included in most IMDs, wherein the IMD communicates with an external device via radio frequency communication. This permits communication with the IMD without requiring presence of a programming head in the proximity of the patient during the communication session. In-office follow-ups are easier and less cumbersome. Further, this also permits a patient's IMD to communicate in virtually any environment without encumbering the patient. For example, a patient may be provided with a home monitor that communicates with the IMD via RF communication and transmits the data to a central server. Thereby, it is, for example, possible to perform long-term monitoring of the cardiac status of the patient.
While providing many benefits, distance telemetry also utilizes scarce power resources, especially during long-term monitoring sessions and there is a need within the art of power management for efficient use of RF telemetry.
Another situation where these scarce power resources may be over exploited leading to severe depletion or drain of the battery may occur during MRI (Magnetic Resonance Imaging) procedures since it is desired to be able to supervise or monitor the patient, e.g. the cardiac status, as well as the functionality of the IMD during such procedures. In theory, the battery may be essentially drained if the patient is monitored during an extensive MRI procedure. MRI is an effective, non-invasive magnetic imaging technique for generating sharp images of the internal anatomy of the human body, which provides efficient means for diagnosing disorders such as neurological and cardiac abnormalities and for spotting tumours and the like. Briefly, the patient is placed within a centre of a large superconducting magnetic that generates a powerful static magnetic field. The static magnetic field causes protons within tissues of the body to align with an axis of the static field. A pulsed radio frequency (RF) magnetic field is then applied causing the protons to begin to precess around the axis of the static field. Pulsed gradient magnetic fields are then applied to cause the protons within selected locations of the body to emit RF signal, which are detected by sensors of the MRI system. Based on the RF signals by the protons, the MRI system then generates a precise image of the selected locations of the body, typically image slices of organs of interest.
Pacemakers and ICDs typically include sensing and detecting circuits for sensing or detecting electrophysiological signal of the heart which signal are used, for example, in the pacing of the patient and/or for monitoring a cardiac status of the patient. As mentioned above, it would be preferable to allow medical devices such as pacemakers or ICDs implanted within the patient to continue to operate in its normal modes during an MRI procedure as long as heating criteria is met, arrhythmias are not induced, unnecessary pacing pulses or shocks are not delivered, and any necessary therapy is not improperly inhibited. That is, it would be desirable to allow the device to continue to monitor the heart of the patient for arrhythmias or other medical conditions even during an MRI procedure. It is also desirable to control the device to transmit monitoring and diagnostic information during the MRI procedure to an external monitoring and control system so that medical personnel can monitor the status of the implanted device and the health of the patient during the MRI scan procedure. In particular, it is of interest to monitor the IEGM of the patient during an MRI scan procedure at the external monitoring and control system to allow the medical personnel to monitor the cardiac status and health of the patient.
An MRI scan procedure normally takes up to two hours and monitoring the cardiac status and health of the patient as well as the functionality of the device thus requires that the RF communication circuits of the device is active and transmits IEGM data during a long period of time. This puts a heavy load on the battery and processing circuits of the device. For example, such long transmission periods draw high currents which deplete the battery and may in fact even damage the battery.
To conclude, power management is a very important aspect in the design and manufacture of IMDs and, in particular, power management for efficient use of RF telemetry during long-term monitoring of the patient, e.g. during MRI procedures, since it uses scarce power resource.